This invention relates to a multiplex transmission system for automotive vehicles in which a multiplex transmission system such as a CSMA/CD system is applied to signal transmission in an automotive vehicle. More particularly, the invention relates to the transmission system of a decentralized multiplex communication network.
The greater use of electronics in automotive vehicles has been accompanied by a number of serious problems, among which are the increased size and complexity of the wire harnesses that interconnect the electronic components. Multiplex communication has become advantageous as a means of solving these problems, particularly in the automobile field. Multiplex communication refers to the time-division multiplexed mixing and transmission of data, from a plurality of nodes, on a single transmission line, and basically employs serial transmission. Nodes are node logical units which control the operation of electrical accessories (e.g., various meters and switches). Each node receives commands from, and outputs commands to, other nodes by communication and is aware of the operation of the other nodes.
In the field of automobiles, multiplex communication networks may be considered as being divided into two classes, one class being complete multiplex, and partial multiplex-type networks, and the other being centralized and decentralized type networks.
The partial multiplex type network mixes a non-multiplex communication portion and a multiplex communication portion. In the multiplex communication portion, switches and loads decentralized in terms of distances are connected by a multiplex transmission unit. However, since separate wiring is required between the multiplex transmission unit and the switches and loads, a drawback is that the numbers of wires is greater, although the overall length of the interconnecting wiring is reduced.
The centralized-type network is one in which a plurality of slave transmission units are connected to a single master transmission unit. With a network having this configuration, a smaller harness diameter is achieved but the entire system is rendered inoperative if the master transmission unit fails. Another shortcoming is that difficulties are encountered in design modification. On the other hand, the decentralized network, though higher in cost, exhibits a number of advantages, such as a much smaller harness diameter, high reliability with regard to partial failure and greater flexibility in terms of design modification. (For example of the latter, see the specification of Japanese Patent Application Laid-Open No. 62-4658).
With regard to this decentralized multiplex communication system, a CSMA/CD system is employed in accordance with an SAE (United States Automobile Technology Group) standardization proposal. According to the transmission system of this standardization proposal, data is transmitted from any node frame by frame, in which each frame possesses a destination address. When the transmitted data is received normally from the transmission line, the multiplex node of the transmission destination designated by the destination address sends back a reception acknowledgment signal (ACK) following the received frame.
The applicant has also proposed a PALNET (protocol for automotive local area network) system which is a further development upon this CSMA/CD system. For example, see the specifications of Japanese Patent Application No. 62-302421 (for which the corresponding U.S. application is U.S. Ser. No. 276,060) and Japanese Patent Application No. 62-302424 (U.S. Ser. No. 276,222). This proposed system is so designed that for every transmission of a single signal frame, the ACK signal is sent back from all active nodes that have received the signal frame. For example, if 16 nodes are connected on a transmission line, the ACK signals from these 16 nodes are transmitted serially on the transmission line. In other words, the ACK signal from each node forms the pattern of a single bit serial. Generally, with the former communication system, only when the frame of one's own address is received does that particular node send back the ACK signal. With the PALNET system, on the other hand, whether or not a received frame is one's own address is discriminated based on the identifier (ID) in the frame. The ACK signal in PALNET does not merely signify that the frame has been received normally but also functions to inform the system at least of the fact that the node which has sent back the ACK signal is active.
When attempting to simplify the signal transmission system of an automobile by multiplex communication, of prime importance is the reliability of the system. Reliability must match the peculiar conditions of the automobile. With the aforementioned CSMA/CD system or PALNET system, the ACK signal will return from a node if the node is non-existent or malfunctioning. Accordingly, when non-return of the ACK signal is detected, determining whether the cause of this is non-existence of the node or failure of the node is theoretically impossible.
In a common LAN (local area network), it is permissible to regard non-return of the ACK signal as being ascribable to non-existence of the node, which may be due to a modification in the system configuration (such as turning off the power supply of a terminal). Such is not the case in an automotive vehicle, however, particularly an automobile. The reason is that in multiplex communication in an automobile there are also nodes which handle electrical accessories (meters, for example) that are essential for operation, and it would be improper to cut such a node off from the network system merely because the ACK signal did not return from the node.
A feature peculiar to automobiles is that an active node changes depending upon the position (OFF, ACC, ON) of the engine key. For example, the node for an air conditioner switch is inactive when the engine key is in the ACC position but active when the key is in the ON position. When the engine key is in the ACC position in a CSMA/CD or PALNET environment, the air conditioner switch node, which will not return the ACK signal in this case, is regarded as being non-existent, just as other nodes which do not return the ACK signal. Accordingly, there is the possibility that a node regarded as being non-existent may exist, despite the fact that it is actually malfunctioning. This is one factor that detracts from the reliability of decentralized multiplex communication. One expedient for eliminating this difficulty would be to supply all nodes with power equally irrespective of the engine key position (OFF, ACC, ON). This would render the nodes active at all times, so that the ACK signal will return from nodes that have not failed, thus making it possible to distinguish between malfunctioning node and active nodes. However, supplying all nodes with power is unrealistic in view of the problem of battery capacity.